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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <!-- <style src="box_model.css"></style> -->
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</head>
<body>
    <div class="demo">B站照抄资料</div>
    <div class="border-box">这是一个边框实例</div>
</body>
</html>